Environmental control systems for aerospace applications have relied on subfreezing heat exchangers for decades. Pressurized air, typically obtained from an engine bleed, is expanded through a turbine which thus extracts work and reduces an outlet temperature low enough to be used as a heat sink for different cooling loads in the system. While lower temperatures are generally desirable, freezing of moisture in the air must be addressed, as frost may buildup on sub-freezing surfaces and impede system operations by clogging airflow and increase resistance of heat transfer.
Subfreezing heat exchangers are known in the art. However as improvements in turbine engine has occurred, the turbine outlet temperature has gone as low as −45 to −65° C. As higher power density systems become relevant, the outlet temperature will continue to be lowered, thus creating additional challenges regarding ice accumulation/frost within the systems. The basic principle of subfreezing heat exchangers is a highly efficient hot side (as hot as possible) and a somewhat inefficient heat transfer on the cold side. Such heat exchangers results in metal temperatures above freezing and thereby prevents ice from building up. Accordingly, it may be advantageous to have systems arranged to reduce or eliminate frost build-up while maintaining desired heat transfer efficiencies.